[BUG] controller.xql and XSLTServlet produces invalid XML

[joewiz]

Describe the bug

As described by @simar0at on exist-open:

I just had a bad and strange experience with some of the lesser popular
setups I think that still are in the docs:

A controller.xql containing this

if (ends-with($exist:resource, ".xml")) then
     <dispatch xmlns="http://exist.sourceforge.net/NS/exist">
         <view>
             <forward servlet="XSLTServlet">
                 <set-attribute name="xslt.stylesheet"
value="{$exist:root}{$exist:controller}/null_transform.xsl"/>
             </forward>
         </view>
         <cache-control cache="no"/>
     </dispatch>

with input xml

<test>
   <strange xml:id="strange" xml:base="./"/>
</test>

and an identity transform null-transform.xsl

<xsl:stylesheet version="3.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
   <xsl:mode on-no-match="shallow-copy" />
</xsl:stylesheet>

yields

<test>
   <strange xmlns:xm="http://www.w3.org/XML/1998/namespace"
xm:id="strange" xm:base="./"/>
</test>

And this is something that a SAX parser for example will refuse to
process again because of this strange binding of the xml namespace to a
new prefix.

Any ideas why this happens?

Expected behavior

The XSLT servlet should not produce XML with an invalid xm namespace prefix for the reserved http://www.w3.org/XML/1998/namespace namespace URI.

To Reproduce

The following XQSuite demonstrates that directly calling transform:transform produces the expected result, while the same transformation processed by the XSLTServlet produces an unexpected result. Here is the test result:

<testsuite package="http://exist-db.org/xquery/test" timestamp="2020-05-15T16:38:07.728-04:00"
    tests="2" failures="0" errors="1" pending="0" time="PT0.024S">
    <testcase name="directly-transform" class="t:directly-transform"/>
    <testcase name="pass-through-controller" class="t:pass-through-controller">
        <error type="err:FODC0005"
            message="Invalid argument to fn:doc or fn:doc-available. exerr:ERROR Error while parsing XML: The prefix &#34;xml&#34; cannot be bound to any namespace other than its usual namespace; neither can the namespace for &#34;xml&#34; be bound to any prefix other than &#34;xml&#34;."
        />
    </testcase>
</testsuite>

The test:

xquery version "3.1";

module namespace t="http://exist-db.org/xquery/test";

declare namespace test="http://exist-db.org/xquery/xqsuite";

declare variable $t:XML := document {
    <test>
       <strange xml:id="foo"/>
    </test>
};

declare variable $t:XSL := document {
    <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="3.0">
        <xsl:mode on-no-match="shallow-copy"/>
    </xsl:stylesheet>
};

declare variable $t:CONTROLLER := ``[xquery version "3.1";

if (ends-with($exist:resource, ".xml")) then
     <dispatch xmlns="http://exist.sourceforge.net/NS/exist">
         <view>
             <forward servlet="XSLTServlet">
                 <set-attribute name="xslt.stylesheet" value="{$exist:root}{$exist:controller}/test.xsl"/>
             </forward>
         </view>
         <cache-control cache="no"/>
     </dispatch>
 
else
    ()
]``;

declare
    %test:setUp
function t:setup() {
    let $testCol := xmldb:create-collection("/db/apps", "test")
    return
        (
            xmldb:store("/db/apps/test", "test.xml", $t:XML),
            xmldb:store("/db/apps/test", "test.xsl", $t:XSL),
            xmldb:store("/db/apps/test", "controller.xql", $t:CONTROLLER),
            sm:chmod(xs:anyURI("/db/apps/test/controller.xql"), "o+x")
        )
};

declare
    %test:tearDown
function t:tearDown() {
    xmldb:remove("/db/apps/test")
};

declare
    %test:assertEquals('foo')
function t:pass-through-controller() {
    doc("http://localhost:8080/exist/apps/test/test.xml")//@xml:id/string()
};

declare
    %test:assertEquals('foo')
function t:directly-transform() {
    let $node-tree := doc("/db/apps/test/test.xml")
    let $stylesheet := doc("/db/apps/test/test.xsl")
    let $parameters := ()
    return
        transform:transform($node-tree, $stylesheet, $parameters)//@xml:id/string()
};

To run this in test, save it to the database as /db/test.xq, and trigger the test via this query: eXist-db/eXide#224 (comment).

Context (please always complete the following information):

• OS: macOS 10.13.6
• eXist-db version: 5.2.0
• Java version: 1.8.0_242 (Zulu 8.44.0.11-CA-macosx) (build 1.8.0_242-b20)

Additional context

• How is eXist-db installed? DMG
• Any custom changes in e.g. conf.xml? none